Position-control device operated by fluid pressure

ABSTRACT

A control device for positioning a body, e.g. a valve, slide or actuator for a stepless hydrostatic transmission in which a control piston is displaced by a control fluid pressure and a working piston is coupled with the actuated member for movement in response to the control pressure independently of external influences upon it. The control piston operates a valve which regulates fluid flow to and from a displacement chamber in which the working piston is shiftable, a spring being disposed within this chamber and acting upon the control piston with a force determined by the position of the working piston.

O Unite States Pate? t 1 1 [111 3,748,967

Richter et a1. July 31, 1973 [54] POSITION-CONTROL DEVICE OPERATED3,511,134 5/1970 Wittren 91/387 BY FLUID PRESSURE 2,947,285 8/1960Baltus et a1. 91/51 3,076,442 2/1963 Raeber 91/51 [75] Inventors: RudolfRichter, l-losbach; Walter 3 5 5,032 1970 Dezelan e1a1, 91/51 Kropp,Obernau, both of Germany [73] Assignee: Linde Aktiengesellschait,Primary Examinerpaul Maslousky wiesbaden Germany AttrneyKarl F. Ross[22] Filed: Nov. 5, 1970 [57] ABSTRACT [21] Appl' 87294 A control devicefor positioning a body, e.g. a valve, slide or actuator for a steplesshydrostatic transmission [30] Foreign Application Priority Data in whicha control piston is displaced by a control fluid Nov. 6, 1969 Germany P19 55 926.3 Pressure and a Working PiSton is Coupled with the atedmember for movement in response to the control [52] US. Cl 91/387,91/461, 92/131 W indePendemly of external influences upon 51 Int. Cl.Flb 13/16 The control piston Operates a valve which regulates 58 Fieldof Search 91/51, 387, 461; fluid flow to and from a displacement chamberin 92/131 which the working piston is shiftable, a spring being disposedwithin this chamber and acting upon the control 55 R f en Cited pistonwith a force determined by the position of the UNITED STATES PATENTS3,003,475 /1961 Rouvalis 91/387 8 Claims, 3 Drawing Figures ll\7 '8 l7 9I I4 l l .-27b I10 7 7 lo I5 141 Ill 1 1 r I I I I I I24 I23 I22 4| I824 2 5 |9 1121 l J 20 40. I1 ,2 20 Q POSITION-CONTROL DEVICE OPERATED BYFLUID PRESSURE FIELD OF THE INVENTION Our present invention relates tofluid-responsive control devices and, more particularly, to afluidresponsive control system for positioning a working member inresponse to a control pressure.

BACKGROUND OF THE INVENTION Hydraulic and pneumatic actuators of manytypes have been proposed heretofore. Such systems include direct-actingarrangements in which the fluid pressure applied to a control pistonacts against the restoring force of a spring to displace the controlpiston and a working member operatively connected therewith. In anothertype of control, the control piston operates a valve which regulatesfluid flow to and from a doubleacting follower cylinder in which aworking piston is shiftable to displace the controlled member. In boththe latter case and the former, frictional resistance of the controlpiston or the working piston and resistance at the actuated member to bedisplaced add to the force of the restoring spring and defeat thedesired linearity or proportionality between a pressure-change incrementat the control piston and a position-change increment at the actuatingmember. Practically all of the fluid-responsive control systems combinethe two arrangements mentioned earlier and, consequently, involve thesame disadvantages. Since the force which adds to the restoring force ofthe spring may be unpredictable or may vary with the position of theactuated member, compensating arrangements in which special springs areprovided, are not always successful in overcoming the disadvantages.

OBJECTS OF THE INVENTION It is, therefore, the principal object of thepresent invention to provide a fluid-responsive control system for theposition of an actuated member which will be independent of the externalinfluences mentioned above and especially resisting and conteractingforces developed within the control system and applied at the actuatedmember.

It is another object of our invention to provide an improved controlsystem in which an actuated member is positioned in response to acontrol pressure randomly and selectively.

A further object of this invention is to increase the versatility andadaptility of a fluid-responsive position control for the purposesdescribed.

SUMMARY OF THE INVENTION These objects and others, which will becomeapparent hereinafter, are attained, in accordance with the presentinvention, with a control system having a control piston subjected tothe control pressure and acting upon a valve controlling the fluid flowto and from a displacement chamber in which a working piston isshiftable, the latter piston being coupled with the member to beactuated. According to the principles of this invention, a restoringspring means acts upon the control piston and its valve in opposition tothe control pressure with a restoring force or a resisting force whichis determined by and dependent upon the position of the working piston.More specifically, the spring means includes a spring seat receivedwithin the displacement chamber and engaged by the working piston fordisplacement thereby, the other spring seat acting upon the valve, e.g.via a force-transmitting pin. In this arrangement, the control piston isautomatically in an equilibrium between the control pressure and thecountervailing force of the spring means, the latter being determined bythe position of the working piston. Each position of the valve andcontrol piston thus corresponds to a particular position of the workingpiston and the position of the working piston is determined solely bythe amplitude of the control pressure and establishes the degree ofcompression of the spring which acts upon the control piston.Furthermore, since the working piston is shiftable in a displacementchamber or cylinder, the position of the working piston is establishedby the liquid volume of this chamber as controlled by the valve operatedby the control piston. External forces, e.g. friction and resistingforces, are taken up by the incompressible fluid within the displacementchamber in the closed position of the valve and hence are not able toaffect the restoring force of the spring. They have, therefore, noinfluence upon the control piston.

According to a more specific feature of this invention, the controlpiston, spring means, valve and working piston are axially aligned in asingle housing which is formed with a bore axially receiving the valveand control piston. The valve and control pistons are constructed as asingle member or body having a piston head at one end which is slidablyreceived in the control cylinder, the latter communicating with a sourceof the control pressure, e.g. an electromagnetic valve. At the other endof this body is provided a valve shoulder so that the head and shoulderare axially spaced apart by an annular groove, the flanks of whichconstitute valve edges of a spool or slide valve regulating the flow offluid to and from an annular chamber formed between these flanks andcommunicating with the displacement chamber. With the control pistonwithdrawn, a clearance is provided enabling the flow of fluid from anannular compartment between the duct communicating with the displacementchamber and the control cylinder into the displacement chamber. In theother position of the valve, fluid is permitted to drain from thedisplacement chamber into a reservoir.

Furthermore, we prefer to provide a forcetransmitting pin between thecontrol piston and the valve seat acting thereon and to dimension thispin so that it functions purely as a mechanical forcetransmitting memberwith the fluid pressure on opposite sides thereof in complete balance.

When the working piston is under load in its normal state, i.e., tendsto be biased against the spring force because nature of the actuatedmember, a single control piston and spring means suffices to determinethe position of the actuated member. On the other hand, when theactuated member has a neutral position in which it is unbiased andtwo-way movement from this position is desired, it is necessary toemploy two-control pistons or a dual-acting control piston in accordancewith the principles set forth above.

According to still another feature of this invention, the spring meansis prestressed, i.e., is received between its spring seats withprecompression, to maintain the working piston in its neutral positionwith a minimum of stored energy being retained in the spring means inthis neutral position. To displace the working piston against the forceof the spring and the resisting forces of the device to. be actuated, weprefer to provide a hydraulic pressure network having a pump or thelike.

DESCRIPTION OF THE DRAWING SPECIFIC DESCRIPTION The pump 1 has itsintake side 1a communicating with a reservoir 2 of hydraulic fluid and apressure 'or discharge side 1b which supplies a hydraulic line 3protected against excess fluid pressure by a pressure relief valve 4shunting the pump 1. Consequently, when no fluid is drawn from line 3,the outflow of the pump 1 is bypassed to the reservoir 2 bypressure-relief valve 4.

A branch 3a of the hydraulic line connects the discharge side of pump 1to randomly or selectively operated valves 5 and 6 constitutingcontrol-pressure regulators, the latter being actuated by means notshown. The actuating means may include energizing circuits for the coils5a and 6a of the valves with arrangement that only one of the pair canbe actuated at any time. In other words, the valves 5 and 6 are actuatedin the alternative.

The valve 5 connects, in one position, a line 7 with the pressure line3a and, in another position, ties the line 7 to a return 40 to thereservoir 2. Line 7 communicates with a control hydraulic cylinder inthe form of a relatively smalLdiameter axial bore of a housing 41. Apiston 10 is axially shiftable in this bore and carries a valve body orslide 1 1 which co-operates with an edge 42 as will be apparenthereinafter. The edge 12 of this valve forms a variable cross-sectionpassage with the edge 42. The valve 11 is received in a valve bore 14axially aligned with the chamber 9.

Annular chamber 15 is provided between the valve chamber 14 and thecontrol cylinder 9 and communicates via a branch 16 with thehigh-pressure side 3a of the hydraulic network.

The head 11 of the valve is received in a chamber 19 which communicatesvia a branch line 43 with the return conduit 20 of the hydraulicnetwork, the conduit 20 opening into the reservoir 2. Axially spacedahead of the piston 10 and valve 1 1, is a compression or throttlingchamber 18 which communicates via a branch 17 with the valve chamber 19.The working piston 21 of the system extends axially into the chamber 18and engages a spring seat 22 which bears against a helical compressionspring 23 surrounding the end of the working piston 21 projecting intothe chamber 18. The spring 23 bears upon a further spring seat 24 which,in turn, can axially press a pressure pin 25 to the right. The pin 25has a head or shoulder 26 which is axially shiftable in a chamber 28 ofsmaller diameter and cross-section than the chambers previouslymentioned. The shoulder 26 divides the chamber 28 into a lefthandcompartment 29 which communicates via line 30 with the return duct 20and a chamber to the righthand side of the shoulder which cancommunicate through an axial bore 27 in this pressure pin and a pair ofaxially spaced radial ports 27a and 27!). A similar structure isprovided at the opposite end of the working piston 21 which may becoupled to the swash plate or swingable cylinder barrel of anaxial-piston pump for controlling the stroke thereof and a transmissionin which the pump is connected. The parts of the lefthand housing 141,which has the same functions as those of the corresponding right-handparts, are indicated with the same numeral preceded by a 1" in thehundreds place.

As previously suggested, the valve 6, which may be operatedalternatively to valve 5 in an exclusive or" sense, whereby only one ofthe valves or neither valve may be operated at any time, alternatelyconnects the line 8 with either the line 30 or the line 40,communicating respectively with the pressure side of the pump and thereservoir. The line 8 delivers fluid 2 or drains the control cylinder109 in which the piston 110 is axially shiftable to displace a valvebody 111. Line 116 connects the annular chamber ahead of the piston 110with the pressure side of the pump while a duct 117 communicates betweenthe chamber surrounding the valve 111 and the throttle chamber 118. Avalve seat 122 bears against the left-hand side of the working piston 21and is urged by a helical compression spring 123 to the right, thespring reacting against a seat 124 which bears upon the pressure pin125.

Of course, the valves 5 and 6, which may be manually actuated, areoperated in see-saw fashion, i.e., when one applies pressure via one ofthe valves 5 or 6 to the respective cylinder 9 or 109, the other valve(5 or 6) connects the other cylinder (9 or 109) to the reservoir, andvice versa.

Upon actuation of the control-pressure regulator 5, we are able to setthe pressure in the line 7 to a randomly selected level and,correspondinglyfthe alternative operation of the valve 6 may establishin line 8 a randomly selected pressure. With a pressure increase in line7, the pressure rises correspondingly in cylinder 9 and tends to shiftthe control piston 10 to the left. In the position of the systemillustrated in the drawing, the control piston 10 has been shifted tothe right by the force-restoring spring means, so that the edge 13 openscommunication between the pressure line 16 and the duct 17. When thecontrol piston 10 is shifted further to the left, edges 12 and 13 closethe opposite ends of the valve chamber 14 so that the fluid mediumneither enters the duct 17 nor is withdrawn therefrom. Still furthermovement of the piston 10 to the left shifts the edge 12 to communicatebetween the duct 17 and the annular chamber 19 and thereby connects theduct 17 with the return line 20 via a branch 43.

In the neutral position of the system, represented in FIG. 1, the springseats 22 and 24 lie against opposite housing portions defining thechamber 18 and the spring 23 is precompressed between them. A movementof the piston 10 to the left is transmitted via a pressure pin 25 to thespring seat 24 and yields a further compression of the spring 23 so thatan opposing movement of the piston 21 must overcome a proportionatelygreater resistance. The pressure pin 25 is exposed, at its left side, tothe pressure in chamber 18 via a compartment 44 which communicates withthe chamber 18 through aperture 45 in the plate 24. So that thispressure does not act upon the pin 25 in addition to the Spring force,the pin 25 is provided with the shoulder 26 whose annular effectivesurface area is dimensioned to equal exactly that of the left-hand endface of the pin 25 and the bore 27 applies the same pressure to thecompartment 28. The opposite side of the shoulder is at the pressure ofthe return line 30, 20. Consequently, the member 25 acts only as amechanical forcetransmitting body.

In operation, there are three essential modes corresponding to anunactuated state of both valves 5 and 6, to the activation of valve 5exclusively and the activation of valve 6 exclusively. When neither ofthe control pressure-regulating valves 5 and 6 are unactuated or one isunactuated while the other is connected to the return 40, the lines 7and 8 remain pressureless and fluid is bypassed to the reservoir 2 bythe valve 4. Members 18 and 118 sustain no elevated pressure, e.g. maybe at the reservoir pressure, so that the working piston 21 controllingthe installation is biased into its control position by the spring 23and 123. The length of piston 21 is therefore selected to correspond tothe distance between the faces engaged thereby of the spring seats 22and 122 when the latter are seated against the end walls of therespective chambers 18 and 118.

When activation of valve 5 results in pressurization of line 7, theselector 9 is pressurized to shift the piston 10 to the left, i.e., inthe direction of the chamber 18. The valve portion 11 of the piston 10establishes communication between the duct 17 and the chamber 19 toconnect this duct with the return line 20. Since, simultaneously, theduct 117 connects the chamber 114 with chamber 118 and chamber 114 isunder the pump pressure delivered to conduit 116, the piston 21 is urgedto the right by the fluid pressure generated in chamber 18 and appliedto the piston and wall 21a through an opening 122a in the seat 122. To acorresponding extent, fluid is driven from chamber 18 and delivered viaduct 17, chamber 19 and line 20 to the reservoir. The spring 23 iscompressed by the spring seat 22 which is carried by the piston 21. As aresult, the spring seat 24 bears with greater force upon the pin 25 and,indirectly, upon the piston 10. As soon as this resistive force equalsthe force applied to the piston 10 in chamber 9, movement stops andequilibrium is reached. This is designed to correspond to a position ofthe valve 11 in which the edges 12 and 13 block the chamber 14 andprevent flow of fluid from and to the chamber 18. The working piston 21thus has a position determined by the control pressure in line 7. Evenif chamber 14 is incompletely closed off, the equilibrium position ofthe member 21 will conform to the pressure applied in chamber 9.

Should the pressure in line 7 be reduced, the piston 10 is biased to theright to enable the pump 1 via lines 3 and 16 and the valve 11 tointroduce fluid through duct 17. The spring 23 is thereby relieved.

As a result, the control piston 10 is shifted further to the left inspite of the reduced pressure in control cylinder 9, until the edges 12and 13 close the bore 14. The working piston 21 then assumes a positionin which the force of spring 23 is balanced by the fluid pressureapplied to the control piston 10 and is immobilized in this position. Itwill be self-evident that all of the other influences upon: the workingpiston 21 are applied to the fluid in chamber 18 and that this fluid issealed within the chamber. The fluid, being substantiallyincompressible, locks the working piston 21 in its position aseffectively as a mechanical lock.

When the valve 6 is actuated to displace the working piston 21 in theopposite direction, the same relationship holds and eventually thepiston 21 assumes a position in which the spring force balances thecontrol pressure applied to the control piston whereupon the valve 111closes communication to and from the displacement chamber 118.

In the embodiment illustrated in FIG. 2, the working piston 221 isprovided at its left-hand side with a stud 31 axially shiftable in thesleeve 32 which, in turn, is axially slidable in the cylinder bore 34.The spring 123 bears against a spring seat or plate 35, resting againstthe sleeve 32 at the end of the latter opposite the end in which theinner bore 36 of the sleeve 32 communicates with a compartmenttherebehind. Moreover, the spring seat 35 has a shoulder 35 in the pathof the stud 31 so that relative axial movement of the stud and thesleeve 32, to shift the stud 31 to the left with respect to the sleeve,will lift the spring seat 35 from the shoulder 32 of the sleeve 32.

In the mutual position of the working piston 221, the sleeve 32 lieswith its right-hand face 32a against the shoulder 37 of the housing.When, however, the working piston 221 is shifted to the left, the fluiddisplacement in chamber 118 applies its rising pressure over the fullcross-section of cylinder bore 34. Since this crosssection is largerthan the cross-section of the working piston 32, a force is producedtending to shift member 221 into its neutral position even when the samefluid pressure is generated in both chambers18 and 118. In the event ofan external force applied to the working piston 221 in the right-handdirection (FIG. 2), the pressure in chamber 18 at the right-hand side ofthe piston 221 rises accordingly. However, at the left-hand side of thepiston 221, the pressure is applied via the stud 31 to the displacementchamber 118 to alter the pressure in the latter precisely in step withthe pressure in chamber 18. Since the cross-section of the stud 31 isless than the cross-section of the working piston 21, the forcedeveloped on the latter overcomes the force at the stud 31 to relocatethe piston 21 in its neutral position. The chamber 38 to which theright-hand side of the sleeve 31 is exposed, is connected to thereservoir by a line 39 and thereby held pressureless. Otherwise, thesystem of FIG. 2 operates as described for the arrangement of FIG. 1.

In FIG. 3, we show another arrangement in accordance with the presentinvention utilizing the principles set forth above with, however, thecontrol system provided solely at the right-hand side of the workingpiston 321. The latter is biased into its neutral position by a pair ofsprings 323 and 323 received within displacement chambers 318 and 318'on opposite sides of a head 321' of the working piston. The spring 323acts upon a seat 324 which rests against a force-transmitting pin 326having a central bore 327 for pressure equalization as previouslydescribed. In this embodiment, however, the control piston 310 has avalve 31] constructed unitarily therewith and provided with spools 311aand 3llb defining four control edges 311e, 311d, 31le and 311],co-operating with annular compartments 314 and 314 communicatingrespectively via duct 317 and 317' with the displacement chambers 318and 318. Pressure chambers 316 and 316 and a reservoir chamber 319 arealso provided and are connected to the pump 301 and the reservoir 302,respectively, or as described in connection with FIG. 1. In thisembodiment as well, a pressure relief valve 304 shunts the intake anddischarge sides of the pump while a valve arrangement 305 is provided tocontrol the pressure in chamber 309 to which the control piston 310 issubject. It will be apparent that depending upon the control pressure inchamber 309, the valve 310 is shifted to effect a correspondingdisplacement of the working piston 321 and the load with the valve 311closing when equilibrium is reached as described. The force of spring323, which may be under precompression, is determined by the position ofthe working piston 321 while the fluid in chambers 318 and 318 locks thepiston when the valve is closed. The piston 321 is thus of thedouble-acting type. The fluid paths in the system of FIG. 3, of course,are similar to those previously described in connection with FIG. 1.

We claim:

1. A fluid-responsive control device, comprising:

housing means defining a displacement chamber;

a working piston slidably received in said housing means and extendinginto said chamber;

a control piston subjected to control-fluid pressure and movably mountedin said housing means; spring means in said housing means and includinga coil-spring arrangement interposed between said working piston andsaid control piston, said coilspring arrangement acting on said controlpiston counter to said control-fluid pressure and with a forcedetermined by the position of said working piston;

and valve means operatively connected with said control piston forregulating fluid flow to and from said chamber for maintaining a body offluid therein adapted to take up external forces applied to said workingpiston in a blocked position of said valve means, said control pistonand said valve means including a spool-valve member provided withaxially spaced spools, said housing means defining a valve bore providedwith annular compartments co-operating with the spools of said valvemember and a control cylinder at one end of said spool-valve member forapplying said control-fluid pressure thereto, said spool-valve member,said coil-spring arrangement and said working piston being axiallyaligned in said housing means, a respective control piston, valve means,coil-spring arrangement and displacement chamber being provided at eachaxial end of said working piston for alternative operation to displacesaid working piston in opposite directions.

2. The device defined in claim 1 wherein said coilspring arrangement ismaintained under a precompression to bias said working piston into aneutral position.

3. A fluid-responsive control device, comprising:

housing means defining a displacement chamber;

a working piston slidably received in said housing means and extendinginto said chamber;

a control piston subjected to control-fluid pressure and movably mountedin said housing means; spring means in said housing means and includinga coil-spring arrangement interposed between said working piston andsaid control piston, said coilspring arrangement acting on said controlpiston counter to said control-fluid pressure and with a forcedetermined by the position of said working piston; valve meansoperatively connected with said control piston for regulating fluid flowto and from said chamber for maintaining a body of fluid therein adaptedto take up external forces applied to said working piston in a blockedposition of said valve means, said control piston and said valve meansincluding a spool-valve member provided with axially spaced spools, saidhousing means defining a valve bore provided with annular compartmentscooperating with the spools of said valve member and a control cylinderat one end of said spool-valve member for applying said control-fluidpressure thereto, said spool-valve member, said coil-spring arrangementand said working piston being axially aligned in said housing means; and

force-transmitting pin interposed between said spool-valve member andsaid coil-spring arrangemerit, said pin being provided with a passagefor equalizing fluid pressure on opposite sides of said 4. The devicedefined in claim 3 wherein said coilspring arrangement includes a coilspring received in said displacement chamber, a first spring seatshiftable in said chamber and resting against said pin and a secondspring seat shiftable in said chamber and resting against said workingpiston, said spring seats receiving said coil spring between them underpre-compression.

5. The device defined in claim 4 wherein said housing means forms a pairof walls on opposite sides of said chamber, said spring seatsrespectively lying against said walls in a neutral position of saidworking piston.

6. The device defined in claim 5, further comprising a fluid-circulatingnetwork including a pump connected with said valve means.

7. The device defined in claim 3 wherein said coil spring arrangement ismaintained under a precompression to bias said working piston into aneutral position.

8. A fluid-responsive control device, comprising:

housing means defining a displacement chamber;

a working piston slidably received in said housing means and extendinginto said chamber;

a control piston subjected to control-fluid pressure and movably mountedin said housing means;

spring means in said housing means and including a spring arrangementinterposed between said working piston and said control piston, saidspring arrangement acting on said control piston counter to saidcontrol-fluid pressure and with a force determined by the position ofsaid working piston; and valve means operatively connected with saidcontrol piston for regulating fluid flow to and from said chamber formaintaining a body of fluid therein adapted to take up external forcesapplied to said working piston in a blocked position of said valvemeans, said control piston and said valve means including a spool-valvemember provided with axially spaced spools, said housing means defininga valve bore provided with compartments co-operating with the spools ofsaid valve member and a control cylinder at one end of said spool-valvemember for applying said control-fluid pressure thereto, saidspool-valve member, said spring arrangement and said working pistonbeing axially aligned in said housing means, a respective controlpiston, valve means, spring arrangement and displacement chamber beingprovided at each axial end of said working piston for alternativeoperation to displace said working piston in opposite directions.

* i III

1. A fluid-responsive control device, comprising: housing means defininga displacement chamber; a working piston slidably received in saidhousing means and extending into said chamber; a control pistonsubjected to control-fluid pressure and movably mounted in said housingmeans; spring means in said housing means and including a coil-springarrangement interposed between said working piston and said controlpiston, said coilspring arrangement acting on said control pistoncounter to said control-fluid pressure and with a force determined bythe position of said working piston; and valve means operativelyconnected with said control piston for regulating fluid flow to and fromsaid chamber for maintaining a body of fluid therein adapted to take upexternal forces applied to said working piston in a blocked position ofsaid valve means, said control piston and said valve means including aspool-valve member provided with axially spaced spools, said housingmeans defining a valve bore provided with annular compartmentsco-operating with the spools of sAid valve member and a control cylinderat one end of said spool-valve member for applying said control-fluidpressure thereto, said spool-valve member, said coil-spring arrangementand said working piston being axially aligned in said housing means, arespective control piston, valve means, coil-spring arrangement anddisplacement chamber being provided at each axial end of said workingpiston for alternative operation to displace said working piston inopposite directions.
 2. The device defined in claim 1 wherein saidcoil-spring arrangement is maintained under a precompression to biassaid working piston into a neutral position.
 3. A fluid-responsivecontrol device, comprising: housing means defining a displacementchamber; a working piston slidably received in said housing means andextending into said chamber; a control piston subjected to control-fluidpressure and movably mounted in said housing means; spring means in saidhousing means and including a coil-spring arrangement interposed betweensaid working piston and said control piston, said coil-springarrangement acting on said control piston counter to said control-fluidpressure and with a force determined by the position of said workingpiston; valve means operatively connected with said control piston forregulating fluid flow to and from said chamber for maintaining a body offluid therein adapted to take up external forces applied to said workingpiston in a blocked position of said valve means, said control pistonand said valve means including a spool-valve member provided withaxially spaced spools, said housing means defining a valve bore providedwith annular compartments co-operating with the spools of said valvemember and a control cylinder at one end of said spool-valve member forapplying said control-fluid pressure thereto, said spool-valve member,said coil-spring arrangement and said working piston being axiallyaligned in said housing means; and a force-transmitting pin interposedbetween said spool-valve member and said coil-spring arrangement, saidpin being provided with a passage for equalizing fluid pressure onopposite sides of said pin.
 4. The device defined in claim 3 whereinsaid coil-spring arrangement includes a coil spring received in saiddisplacement chamber, a first spring seat shiftable in said chamber andresting against said pin and a second spring seat shiftable in saidchamber and resting against said working piston, said spring seatsreceiving said coil spring between them under pre-compression.
 5. Thedevice defined in claim 4 wherein said housing means forms a pair ofwalls on opposite sides of said chamber, said spring seats respectivelylying against said walls in a neutral position of said working piston.6. The device defined in claim 5, further comprising a fluid-circulatingnetwork including a pump connected with said valve means.
 7. The devicedefined in claim 3 wherein said coil-spring arrangement is maintainedunder a precompression to bias said working piston into a neutralposition.
 8. A fluid-responsive control device, comprising: housingmeans defining a displacement chamber; a working piston slidablyreceived in said housing means and extending into said chamber; acontrol piston subjected to control-fluid pressure and movably mountedin said housing means; spring means in said housing means and includinga spring arrangement interposed between said working piston and saidcontrol piston, said spring arrangement acting on said control pistoncounter to said control-fluid pressure and with a force determined bythe position of said working piston; and valve means operativelyconnected with said control piston for regulating fluid flow to and fromsaid chamber for maintaining a body of fluid therein adapted to take upexternal forces applied to said working piston in a blocked position ofsaid valve means, said control piston and said valve means including aspool-valve member provided with axially spaced spools, said housingmeans defining a valve bore provided with compartments co-operating withthe spools of said valve member and a control cylinder at one end ofsaid spool-valve member for applying said control-fluid pressurethereto, said spool-valve member, said spring arrangement and saidworking piston being axially aligned in said housing means, a respectivecontrol piston, valve means, spring arrangement and displacement chamberbeing provided at each axial end of said working piston for alternativeoperation to displace said working piston in opposite directions.